A power distribution system is the interface between the power transmission network and the electricity end-customers. The power distribution system can include a number of primary substations which are connected to secondary substations via power lines and switches. The primary substations contain transformers that reduce the voltage from the HV (high voltage) level of the transmission or sub-transmission grid down to MV (medium voltage) levels suitable for regional transmission. Distribution level network operation involves secondary devices interacting with primary equipment of the substations and power lines such as switches, tap changers, capacitor banks and the like. A Distribution Automation (DA) system responsible for controlling, protecting, measuring and monitoring includes the secondary devices interconnected in a Substation Automation (SA) communication network and interacting with the primary devices via a process interface.
In automated power transmission and distribution, the secondary devices include microprocessor-based programmable electronic devices or digital relays, also termed Intelligent Electronic Devices (IED), that are capable of sending control signals to switching devices, such as circuit breakers and disconnectors. Most digital relays in use today combine protection, control, monitoring, communication, power quality monitoring, and metering capabilities. The protection functions supported by a digital relay can include time delay and instantaneous over-current functions for phase and ground elements, sequence directional over-current functions, reclosing functions, over- and under-frequency protection functions, and over- and under-voltage protection functions. Digital relay devices also support various metering functions, monitoring of voltage sags, swells and interruptions, fault location algorithms, and oscillographic record storage. In addition, digital relay devices can be configured locally using the front panel of the digital relay or remotely using a settings software tool. In many cases, the required protection functionality is preloaded into the relay device, and later enabled and parameterized with a tool connected to the relay device.
As outlined above, a basic function of a protection relay is to protect electrical equipment by tripping a circuit breaker and interrupting a power line in case of over current or earth fault situations. The tripping signal on behalf of a trip coil or other actuator of the circuit breaker is generated by the protection relay, for example, when the measured current in the line exceeds a nominal or setting value for a predefined time period. For instance, fault currents above twenty times the rated current of a current transformer may cause the protection relay to release a trip signal with 40° msec delay.
For over current and earth fault protection of individual power lines, static auxiliary-powered protection relays with a station battery supplying power for the relay electronics may be used. However, in certain situations such as Ring Main Units (RMU) installations in urban areas, auxiliary-powered protection relays may not be acceptable, and so-called self-powered or self-supplied relays are used instead. In this case, the energy required by the relay electronics circuit as well as the energy released to the trip coil is supplied by current sensing transformers. An optimal balance of the micro-controller hardware design and the associated software is desired, and the self-supplied protection relay may be customized by a user through mechanical binary or Dual In-line Package (DIP) switches for parameter setting, or alternatively through battery-powered alphanumeric LCD based Human Machine Interfaces (HMI).